Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on xa_store() failure

[Michal Luczaj <mhal@rbox.co>]

On 7/30/24 17:56, Will Deacon wrote:
> If the xa_store() fails in kvm_vm_ioctl_create_vcpu() then we shouldn't
> drop the reference to the 'struct kvm' because the vCPU fd has been
> installed and will take care of the refcounting.
> 
> This was found by inspection, but forcing the xa_store() to fail
> confirms the problem:
> 
>  | Unable to handle kernel paging request at virtual address ffff800080ecd960
>  | Call trace:
>  |  _raw_spin_lock_irq+0x2c/0x70
>  |  kvm_irqfd_release+0x24/0xa0
>  |  kvm_vm_release+0x1c/0x38
>  |  __fput+0x88/0x2ec
>  |  ____fput+0x10/0x1c
>  |  task_work_run+0xb0/0xd4
>  |  do_exit+0x210/0x854
>  |  do_group_exit+0x70/0x98
>  |  get_signal+0x6b0/0x73c
>  |  do_signal+0xa4/0x11e8
>  |  do_notify_resume+0x60/0x12c
>  |  el0_svc+0x64/0x68
>  |  el0t_64_sync_handler+0x84/0xfc
>  |  el0t_64_sync+0x190/0x194
>  | Code: b9000909 d503201f 2a1f03e1 52800028 (88e17c08)
> 
> Add a new label to the error path so that we can branch directly to the
> xa_release() if the xa_store() fails.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Michal Luczaj <mhal@rbox.co>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  virt/kvm/kvm_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d0788d0a72cc..b80dd8cead8c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4293,7 +4293,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  
>  	if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
>  		r = -EINVAL;
> -		goto kvm_put_xa_release;
> +		goto err_xa_release;
>  	}
>  
>  	/*
> @@ -4310,6 +4310,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  
>  kvm_put_xa_release:
>  	kvm_put_kvm_no_destroy(kvm);
> +err_xa_release:
>  	xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
>  unlock_vcpu_destroy:
>  	mutex_unlock(&kvm->lock);

My bad for neglecting the "impossible" path. Thanks for the fix.

I wonder if it's complete. If we really want to consider the possibility of
this xa_store() failing, then keeping vCPU fd installed and calling
kmem_cache_free(kvm_vcpu_cache, vcpu) on the error path looks wrong.